The use of ophthalmic lenses for the correction of ametropia is well known. For example, multifocal lenses, such as progressive addition lenses (“PALs”), are used for the treatment of presbyopia. Typically, the progressive surface of a PAL has three zones, one zone each of distance, intermediate, and near vision power. The distance vision power zone permits viewing of objects located at more than 70 cm from the viewer, while the intermediate zone permits viewing of objects at between 45 and 70 cm, such as a computer monitor and the near vision power zone permits viewing of objects at less than about 45 cm. The boundaries between each of these zones are typically continuous and blended so that the image jump between the zones is minimized. Thus, the refractive power is provided in a gradual, continuous progression of vertically increasing dioptric power from far to near focus, or top to bottom of the lens.
In conventional PALs, the near vision power zone extends to the bottom edge, or inferior-most portion, of the lens. Because of this, viewing of objects through this portion of the lens, which objects are outside of the near vision range, are problematic. For example, the ground or stairs when viewed through this zone will appear blurred to the lens wearer and the wearer must remove the lenses or stoop to see the ground or stairs clearly.
One method of overcoming this problem is to provide a near vision zone of a specified length and to decrease the add power in the portion of the lens inferior to the near vision power zone. Typically, the rate of decrease will be very slow so that significant unwanted astigmatism, meaning astigmatism introduced or caused by one or more of the lens' surfaces, does not result. However, this does not achieve the objective of providing clear visualization of the ground or stairs. If the rate of decrease is faster, in addition to significant unwanted astigmatism, prism and magnification changes cause objects viewed through this zone to shift and change in size.
Therefore, a need exists for a progressive addition lens that provides a zone inferior to the near vision zone that overcomes these disadvantages.